This invention relates to the protection and miniaturization of the circuits placed in a shield case.
Shield cases are used to house several kinds of circuits such as voltage controlled oscillators (VCOs) used in the production of car telephones and other cordless telephones. As the cost of producing cordless telephones decreases and their popularity increases, a demand for smaller, more convenient cordless telephones arises. One of the major problems of miniaturizing cordless telephones is the reduction in size of the VCOs which is caused by the current design of the shield case insulation cover.
FIG. 5 depicts one of the earlier conventional designs for VCO units used in cordless telephones. This VCO unit as an oscillator-modulator (not illustrated in the figure) located on the inside of the box-type prior art shield case 1. Prior art shield case 1 was attached to mother board 3 by inserting four (4) prior art installation legs 2, formed on the bottom edge of the shield case 1, into the installation holes 4 in the mother board 3. During installation a prior art insulator 7, made of mica or other insulating material, was inserted between the prior art shield case 1 and the mother board 3 by aligning the holes in prior art insulator 7 with the installation holes in mother board 3, then inserting the prior art installation legs 2 on prior art shield case 1 through the prior art insulator 7 and into the mother board 3. The purpose of prior art insulator 7 is to prevent short circuits between the pattern circuit wiring 5 or the through-hole 6 and the prior art shield case 1.
The major problem with this early form of VCO was that the bottom of the shield case remained open and the interior exposed to contamination until the time it was assembled to the mother board. This allowed impurities to enter the shield case and affect the performance of the VCO. This problem was addressed by the introduction of an insulation cover which could be attached to the bottom of the shield case prior to installation. This prior art solution is shown in FIG. 6.
FIG. 6 depicts the current use of intermediary seats 8 located between the prior art installation legs 2 and the bottom edge of shield case 1. The intermediary seats 8 are somewhat larger than the installation holes 4 in the mother board 3, thereby acting both as a platform which causes a gap between the mother board 3 and the bottom edge of prior art shield cover 1, and as a wedge used to hold prior art insulator cover 9 onto the bottom of the shield case prior to installation onto mother board 3. As shown in FIG. 9, an insulation cover 9 is placed on the bottom of the prior art shield case 1, through which pass the prior art installation legs 2 and intermediary seats 8, which extends out from the bottom edge of the shield case 1 through the gap caused by intermediary seats 8.
The problem arising from the use of prior art insulator covers is that the cover can be easily bent away from the bottom of the shield case, shown in FIG. 9, thereby allowing foreign matter to enter the shield case and affect the performance of the VCO. A second problem which arises is that when the shield case is installed on the mother board, a portion of the insulation sticks out from beneath the shield case forming a perimeter. This perimeter covers space on the mother board which could be eliminated to facilitate miniaturization. Also, because the shield case design must account for the perimeter in order to attach the insulation to the shield case, the miniaturization of the shield case itself is limited.